N-channel metal oxide semiconductor (NMOS) transistors have been frequently used in advanced complementary metal oxide semiconductor (CMOS) technology. It is important to modulate the effective work function in the metal gates of the NMOS transistors.
Conventional approach is to dispose multiple layers of work function metal films in the metal gates. In order to achieve different work function in different metal gates, very often, more than one work function metal film is used. The thickness of the metal gates therefore increases and varies according to the function of the metal gates. The increased thickness in metal gates is not desired in device scaling.